It has become general to use WAN (Wide Area Network) utilizing IP-VPN technology or the like, as a communication network between global bases.
In the case where a certain terminal at a certain base communicates with a certain terminal at a different overseas base, communication is performed via a line connecting one's own base LAN with a domestic WAN, a line connecting a domestic WAN with an overseas WAN, and a line connecting an overseas WAN with another base LAN. These lines are limited in available bandwidth width, based on contracted bandwidth.
It is general to use TCP in communication between terminals. In TCP communication, an amount of data already received by a receiving terminal is fed back to be notified to a transmitting terminal, the data being sent by the transmitting terminal. When the amount of the already received data to be fed back to be notified stops increasing, the transmitting terminal judges it as discard detection.
Further, the transmitting terminal manages a parameter called a window size (a transmittable data size even when the fact of reception is not acknowledged from the receiving terminal), and changes the window size in response to RTT (Round Trip Time) or presence or absence of discard detection.
By decreasing the window size when the network is judged busy during increasing in RTT or in discard detection, the transmission bandwidth is decreased indirectly to avoid network congestion. In addition, by increasing the window size when decreasing in RTT or absence of discard detection, the network is judged vacant and the transmission bandwidth is increased indirectly so that the line bandwidth of the network is effectively utilized. In this way, in communication using TCP, the transmission bandwidth is significantly influenced by RTT and discard rate.
As technology similar to communication using TCP, there is also communication using ABR of ATM. In this communication, in addition to RTT or discard, the receiving bandwidth is fed back to be notified from the receiving terminal to the transmitting terminal.
In addition to RTT and discard detection, there is also technology of direct bandwidth control using a connection number (PATENT LITERATURE 1).